1. Field of the Invention
The present invention generally relates to a color separation device that separates linearly polarized white light into three primary colors, an imaging optical engine that generates a color image (color image light) using such color separation device, and a projection apparatus using such imaging optical engine.
2. Description of the Related Art
Recently, “liquid crystal projectors” that project images displayed on liquid crystal panels, etc. onto display media such as a screen, etc. in a larger scale are widely used for displaying video reproduction images and computer data, etc. In particular, “three-panel liquid crystal projectors” used for a color image presentation are widely used since the color images can be displayed thereby with a high resolution.
The liquid crystal panels are generally referred to as “light valves” and they can change the polarization direction of output light with respect to linearly polarized input light on a pixel basis.
Meanwhile, “wavelength selective retarders” that can provide “a half-wavelength retardation” between ordinary light and extraordinary light in the linearly polarized light in a desired waveband are known. The half-wavelength retardation can be provided to “light in the waveband that can be arbitrarily determined as a design condition.” Therefore, for example, when p-polarized light is made incident on the wavelength selective retarders, only certain waveband is s-polarized and the other wavebands are remained p-polarized.
Recently, three-panel liquid crystal projectors using such wavelength selective retarders have been proposed (see for example, Japanese Laid-Open Patent Application No. 2000-284228).
FIG. 1 shows a schematic configuration of one example of a projection apparatus mounted on a three-panel liquid crystal projector according to the related art. The projection apparatus includes a light source part 1 having a lamp 11 and a polarization plate 12, a color separation part 2 having a polarization beam splitter 21 for color separation, a narrowband phase difference plate (wavelength selective retarder) 22 for the red component, and a narrowband phase difference plate (wavelength selective retarder) 23 for the blue component, a GB (green/blue components) modification part 3 having a polarization beam splitter 31 for the green and blue components, a liquid crystal panel (reflective optical modulator) 32 for the blue component, and a liquid crystal panel (reflective optical modulator) 33 for the green component, a red component modification part 4 having a polarization beam splitter 41 for the red component and a liquid crystal panel (reflective optical modulator) 42 for the red component, a color combining part 5 having a polarization beam splitter 51 for color combining and a narrowband phase difference plate (wavelength selective retarder) 52 for the green component, and a projection optical system part 6.
In such a three-panel liquid crystal projector, the linearly polarized white light is incident on the first wavelength selective retarder (described as “narrowband phase difference plate” in the cited document) 22. Among the lights that passed through the first wavelength selective retarder 22, waveband light (A) having its polarization direction changed and waveband light (NA) not having its polarization direction changed are separated to have different optical paths respectively by the polarization beam splitter 21. The waveband light (A) having its polarization direction changed is applied to the first light valve (mentioned as “reflective light modulator” in the cited document) 42. The waveband light (NA) not having its polarization direction changed is further separated into waveband light (B) and waveband light (C), each of the waveband lights (B, C) having a different primary color, by the second wavelength selective retarder 23 and the polarization beam splitter 31. The waveband light (B) is applied to the second light valve 32 and the waveband light (C) is applied to the third light valve 33. After combining image lights (LB, LC) that are modulated according to image signals and which are reflected from the second light valve 32 and the third light valve 33 by the second polarization beam splitter 31, the polarization directions of the image lights (LB, LC) are made coincident with one another by the third waveband selective retarder 52. Whereas, image light reflected from the first light valve 42 (LA) is combined with the previously combined image lights LB, LC by the third polarization beam splitter 51. The combined color image light is incident on the projection optical system 6, which projects the combined color image light onto a screen (not shown) so as to display a color image.
As for the wavelength selective retarders used in such a projection apparatus, an element formed by stacking phase difference plates that make use of double reflection of light disclosed in Japanese patent No. 130537 (Japanese Laid-Open Application No. 11-504441) may be used.
In the projection apparatus disclosed in the Japanese Laid-Open Patent Application No. 2000-284228, image light formed by liquid crystal panels (light valves) for respective primary colors are directed to a projection lens by passing through polarization beam prisms or dichroic prisms, etc. In such a projection apparatus, the projection lens is designed so that the projection magnification, etc. with respect to each liquid crystal panel are precisely controlled in order to make the back focal distance from the projection lens to each liquid crystal panel equal. However, when imaging optical paths having wavelength selective retarders made from materials that are different from glass material of the prisms, in other words, made from material having refraction indices different from that of the prism, and imaging optical paths without those wavelength selective retarders are used together, color image quality is adversely affected. In addition, it is difficult to devise a design for the projection lens that keeps a high level of performance.
Further, as another example of the related art, a reflective liquid crystal projector device is proposed in Japanese Laid-Open Patent Application No. 2001-281614. The proposed projector device is characterized in that an optical engine part in a projection unit of the proposed projector device is integrally configured, as a color separation/combining part, from a first color separation part, an entry and exit directions change part, a second color separation part, a color combining part, and a support part and in that a polarization direction-rotating member (narrowband phase difference plate corresponding to the wavelength selective retarder) is stacked onto the color separation parts.
The projection unit of the proposed projector device is configured from, for example, as shown in FIG. 2, a light source part 301 having a lamp 311 and a polarization plate 312, a color separation part 302 having a polarization separation surface 321 for color separation, a narrowband phase difference plate 322 for red component, and a narrowband phase difference plate 323 for blue component, a GB (green/blue components) modulation part 303 having a polarization separation surface 331 for green component and blue component, a liquid crystal panel 332 for the green component, and a liquid crystal panel 333 for the blue component, and a R (red component) modulation part 304 having a polarization separation surface 341 for the red component and a liquid crystal panel 342 for the red component, a color combining part 305 having a polarization separation surface 351 for color combining and a narrowband phase difference plate 352 for the blue component, a positioning support 306, and a projection optical system 307.
However, in such projector device configured as shown in FIG. 2, at least three sets of polarization direction-rotating members (narrowband phase difference plates) 322, 323, 352 are necessary. Not only does it cause an increase in cost due to an increase in the number of components but it may also generate a risk of reducing the performance level of the projector device when, for example, assembly error accumulates during manufacture. Also, as shown in FIG. 2, image light from the liquid crystal panel 332 for the green component or the liquid crystal panel 333 for the blue component passes through the polarization direction-rotating member (narrowband phase difference plate for the blue component) 352 at an angle of 45 degrees. This means that the image light passes through the polarization direction-rotating member 352 in the form of a parallel plate diagonally and this leads to deviation in the optical path, etc. Further, since the image light formed by the liquid crystal panel 342 for the red component dose not pass through any of the polarization direction-rotating members, optical path length of the red image light and that of blue and green image light are different, and this leads to quality reduction in the composite color image.